Such brake systems having stacks of disks (generally made of carbon) are in widespread use on aircraft of recent design. Generally, the stator central portion is provided with a brake collar equipped with a plurality of sets of pistons and, rigidly secured to said collar, a torsion tube. The stator portion is supported on the axle in question firstly at the brake collar, and secondly at a transverse annular web present inside the torsion tube. A centering bearing is then interposed between the annular web and the axle. Reference may be made, for example, to Documents U.S. Pat. No. 6,003,641 and EP-A-0 990 813.
In conventional landing gear, a landing gear leg is provided together with a telescopic rod to the end of which a wheel lever carrying a pair of wheels is hinged. The wheel lever is thus hinge mounted on a yoke secured to the bottom end of the telescopic rod, and a bar is interposed between the brake collar and one of the branches of the above-mentioned yoke for the purpose of taking up the torsion forces generated during braking.
When the brake system is actuated by the pilot, the axles are subjected to simple flexing in two perpendicular directions, which corresponds to two coupled characteristic modes. In addition, the structure which surrounds the axle is also subjected to flexing forces of the same type, generating dynamic deformations of the same type, which deformations take place with a small amount of phase delay relative to the deformations of the axle. The two coupled modes combine to generate an unstable mode, and specialists know that if the two modes have frequencies that are close to each other, the probability of the two modes combining into an unstable mode is high. Thus, an approach has emerged aiming to move the characteristic modes apart to so as to prevent the resulting unstable mode from appearing.
Proposals have been made to act hydraulically on the brake collar by throttling hydraulic fluid to generate damping. However, in addition to being complex, such a solution is unsatisfactory in terms of reliability, in particular in the event of hydraulic failure.
Attempts have also been made to act on the stiffness of the torsion tube by using a sandwich of different materials. That theoretically makes it possible to adapt the axial stiffness of the torsion tube or of the surrounding structure. However, in that approach, it is extremely difficult to control the stiffnesses reliably in different directions.
A more advantageous solution has recently been proposed, aiming to make use of the presence of the transverse annular web by constraining said web to have a particular and non-uniform behavior. Unlike the other conventional designs in which the transverse annular web is uniformly perforated with circular holes, that solution has focused on providing slots which are circumferentially elongate, which slots generate asymmetry. The axle is then retained on a portion of circumference with rigidity that is larger in one zone than in the residual zone associated with the presence of circumferential slots. By appropriately selecting the angular extent of the circumferential slots and their positions relative to the main axes, that solution appears to make it possible to lower the low-frequency mode. However, such an approach suffers from the drawback of weakening the transverse annular web. In addition, modifying radial stiffnesses also induces a modification in axial stiffness, which is also unfavorable structurally.